Revealing emplacement dynamics of a simple flood basalt eruption unit using systematic compositional heterogeneities

Intra-lava geochemical variations resulting from subtle changes in magma composition are used here to provide insights into the spatial-temporal development of large basalt lava flow fields. Recognition that flood basalt lavas are emplaced by inflation processes, akin to modern pāhoehoe lava, provides a spatial and temporal framework, both vertically at single locations and laterally between locations, to examine lava flow emplacement and lava flow field development. Assuming the lava inflation model, we combine detailed field mapping with analysis of compositional profiles across a single flow field to determine the internal spatio-temporal development of the Palouse Falls flow field – a lava produced by an individual Columbia River flood basalt eruption. Geochemical analyses of samples from constituent lobes of the Palouse Falls lava field demonstrate that systematic compositional whole-rock variations can be traced throughout the flow field from the area of the vent to distal limits. Chemical heterogeneity within individual lava lobes (and outcrops) show an increase from lava crusts to cores, e.g., MgO = 3.24 to 4.23 wt%, Fe2O3 = 14.71 to 16.05 wt%, Cr = 29 to 52 ppm, and TiO2 = 2.83 to 3.14 wt%. This is accompanied by a decrease in incompatible elements, e.g., Y = 46.1 to 43.4 ppm, Zr = 207 to 172 ppm, and V = 397 to 367 ppm. Systematic compositional variations from the source to distal areas are observed through constituent lobes of the Palouse Falls flow field. However, compositional heterogeneity in any one lobe appears less variable in the middle of the flow field, as compared to more proximal and distal margins. Excursions from the general progressive trend from vent to distal limits are also observed and may reflect lateral spread of the flow field during emplacement, resulting in the juxtaposition of lobes of different composition. Transport of magma through connected sheet lobe cores, acting as internal flow pathways to reach the flow front, is interpreted as the method of lava transport. Additionally, it can explain the general paucity of lava tubes within flood basalt provinces. In general, flow field development by a network of lava lobes may account for the occurrence of compositionally similar glasses noted at the proximal and distal ends of some flood basalt lavas.